To ensure quality and accuracy during imaging in an internal drum imaging system, such as an imagesetter, platesetter, or the like, the imaging beam is often shielded as it passes between the optical carriage and the imaging surface. In the past, this has been accomplished in a number of ways. For example, as disclosed in U.S. Pat. No. 5,589,973, assigned to the Agfa Division of Bayer Corporation, incorporated herein by reference, a radial shield pair, or "clam shell," is used to shield the imaging beam. The imaging beam is protected between the two halves of the shell from thermal currents and various other conditions which could cause deleterious beam deflections. The shell also serves to contain the imaging beam to prevent stray reflections which could cause unwanted exposure of the imaging material.
This type of shield is typically produced from materials such as aluminum, since it needs to be both rigid and lightweight. The rigidity is required so that a reasonably small gap can be maintained between the shells of the shield for passage of the beam without the shells moving into the beam path. The lightweight construction of the shield is required because the shield is generally attached to the optical carriage which traverses the drum axially while imaging. In general, adding mass to the carriage degrades its performance.
A further example of such a shield may be found in U.S. Pat. No. 5,671,005, assigned to the Agfa Division, Bayer Corporation, incorporated herein by reference.
In another configuration of an internal drum imaging system, an applicator assembly is used for moving imaging materials into the drum and/or "ironing" the imaging materials against the drum surface. In order to avoid interference between the shield and the applicator assembly, the optical carriage and attached shield must be moved axially outside the drum area serviced by the applicator assembly. Unfortunately, this decreases the throughput of the imaging system because additional time is required to move the optical carriage and attached shield out of the drum away from the applicator assembly during the loading of imaging material, and to return the optical carriage and attached shield to an imaging position within the drum. Additionally, the size of the drum and/or housing of the imaging system must be considerably larger to accommodate the extra travel distance of the optical carriage, thereby increasing the overall size and weight of the imaging system.